This invention is related to voltage regulators and more particularly to a circuit for limiting output current in a flyback switching regulator.
Flyback switching regulators are often selected because they provide good output voltage regulation over relatively large variations in input voltage. However, since the peak current in the secondary of a flyback transformer increases while its average value stays the same with a drop in primary voltage, the primary current increases both in magnitude and duration, causing current limiting to be very difficult to achieve. One form of current limiting has been to place a resistor in series with the primary coil for sensing the current through the primary. If the output of the flyback transformer circuit is shorted, and the circuit is current limited by measuring the primary current, the secondary current will increase. If the regulator is designed for a large input voltage swing, the secondary current could increase several times its rating. This current limit circuit operates by turning off the primary current at a predetermined level and back on at a predetermined point in the cycle. As long as the short remains, the very large secondary circuit current will flow.
For small changes in input voltage, it is feasible to use a sensing resistor in the primary circuit for limiting output current since the current set point can be based upon minimum input voltage variations. However, for large variations in input voltage, the current set point must be selected such that the output current can grow quite large in a short circuit fault situation.
Accordingly, it is an object of the present invention to provide a switching voltage regulator having improved short circuit current limiting characteristics.
It is another object of the present invention to provide a switching regulator current limit circuit for limiting overload output currents in flyback switching regulators.
It is a further object of the present invention to provide a series switching regulator employing a feedback coil on a flyback transformer to sense the output voltage and modify the current limit point to a lower level.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.